Flower strips, crop management and landscape composition effects on two aphid species and their natural enemies in faba bean

Wildflower strips and biodiversity-friendly management are promoted as ways of enhancing biodiversity and natural pest control in agroecosystems. Their efficacy for providing resources and shelters for natural enemies has been well studied, but little is known about how wildflower strip efficacy is mediated by local field management and landscape composition. In this study, we explored the effects of floral resources in perennial flower strips on faba bean aphids and their antagonists in contrasting cropping systems and landscapes. We surveyed 13 faba bean crops in northern France in 2019, and 19 in 2020. We sampled aphids (Aphis fabae and Acyrthosiphon pisum) and their antagonists (Syrphidae, Coccinellidae and mummy-forming parasitoids) on three dates during the spring, by visual observation. We assessed the floral resources provided by wildflower strips and spontaneous field margins and collected data on field management practices and land use in the surrounding landscape. We found that the percentage of flowering species in field edges providing accessible nectar to natural enemies had a positive effect on the abundance of Syrphidae and aphid mummies on faba bean plants. The positive effect of nectar resources on Syrphidae and aphid mummies decreased with increases in the proportion of semi-natural habitats in the landscape, but no interaction was observed with local management techniques. The growth rate of Aphis fabae and Acyrthosiphon pisum populations were unaffected by their antagonists. The growth rate of A. fabae populations was driven principally by local management practices (i.e. insecticide spraying and tillage). Conversely, the growth rate of A. pisum populations was driven principally by landscape composition (i.e. the proportion of semi-natural habitats and leguminous plants) and was negatively related to insecticide use. Nevertheless, insecticide spraying did not prevent increases in the growth rates of both aphid species, highlighting the need for alternative methods for controlling aphid populations.